1. Field of the Invention
The present invention relates to a dielectric ceramic composition having reduction resistance and an electronic device, such as a multilayer ceramic capacitor, using the dielectric ceramic composition.
2. Description of the Related Art
A multilayer ceramic capacitor as an electronic device is widely used as a highly reliable compact electronic device having a large capacity and by a large number in one electronic apparatus. In recent years, as the apparatuses become more compact and higher in performance, demands for a multilayer ceramic capacitor to be more compact with a larger capacity, low at cost and higher in reliability have become still stronger.
A multilayer ceramic capacitor is normally produced by using internal electrode layer paste and dielectric layer paste, stacking them by using a sheet method or a printing method, etc. and co-firing internal electrode layers and dielectric layers in an obtained stacked body.
As a conductive material of the internal electrode layers, Pd and a Pd alloy are generally used, but due to the high price thereof, relatively inexpensive base metals, such as Ni and a Ni alloy, have come to be used. When using a base metal as a conductive material of internal electrode layers, the internal electrode layers are oxidized when fired in the air. Therefore, co-firing of the dielectric layers and internal electrode layers has to be performed in a reducing atmosphere. However, when fired in a reducing atmosphere, the dielectric layers are reduced and the specific resistance declines. Thus, reduction-resistant dielectric materials have been developed.
However, a multilayer ceramic capacitor using a reduction-resistant dielectric material has disadvantages that a decline of IR (insulation resistance) due to an application of an electric field is remarkable, the IR lifetime is short and the reliability is low.
Capacitors are also required to have preferable capacity-temperature characteristics and, particularly, required to have a flat temperature characteristic of the capacity under a severe condition depending on the use object. In recent years, multilayer ceramic capacitors have come to be used in a variety of electronic apparatuses, such as an engine electronic control unit (ECU) to be installed in an engine room of vehicles, a crank angle sensor and an Anti Lock Brake System (ABS) module. Since these electronic apparatuses are for stable engine controlling, drive controlling and brake controlling, preferable temperature stability of the circuit is required.
Environments of using the electronic apparatuses are expected to become about −20° C. or lower in winter in cold climates and about +130° C. or higher after the engine starts in summer. Recently, there is a tendency of reducing a wire harness for connecting the electronic apparatus and an apparatus to be controlled thereby and the electronic apparatus may be provided outside of a vehicle, therefore, environments for the electronic devices have been getting more severe. Accordingly, a capacitor used by the electronic apparatuses is required to have flat temperature characteristics in a wide temperature range. Specifically, it is not sufficient when the capacity-temperature characteristics only satisfy the X7R characteristic of EIA standard (−55 to 125° C. and ΔC/C=within ±15%), and a dielectric ceramic composition satisfying the X8R characteristics of EIA standard (−55 to 150° C. and ΔC/C=within ±15%) is required.
As a dielectric ceramic composition satisfying the X8R characteristics, for example, the Japanese Unexamined Patent Publication No. 2004-214539 discloses a dielectric ceramic composition having a predetermined composition and unevenness of average Ca concentration in crystal grains between the crystal grains is 5% or higher when expressed in a CV value. The dielectric ceramic composition described in the publication satisfies the X8R characteristics, and capacity-temperature characteristic exhibited a usable property in a range of −55 to 150° C. However, the dielectric ceramic composition in the publication has a disadvantage of having poor insulation resistance (IR) temperature dependency and, for example, the insulation resistance (IR) declines in a high temperature environment, so that it becomes difficult to use it under high temperature. Furthermore, the dielectric ceramic composition in this publication has a disadvantage of having poor TC bias characteristics.